Hong Kong servers boast a unique geographical advantage in international internet access, making them widely used in business scenarios such as cross-border e-commerce, live streaming, and overseas acceleration. However, during routine maintenance, many users experience a sudden inability to resolve their previously stable DNS resolution after upgrading their Hong Kong servers. This can lead to website inaccessibility, software connection failures, and even slow remote desktop responses.

I. Common Causes of DNS Failure

1. DNS configuration reset or loss. The system upgrade process may reset the network adapter configuration. Especially on servers with static IP addresses, manually configured DNS server addresses may be reset to blank or default routes. If a valid DNS server is not configured after the upgrade, domain name resolution will be impossible.

2. Changes in network firewall policies. After the upgrade, the system may automatically enable stricter security policies, particularly those in Windows' Advanced Firewall or Linux's firewalld. If outbound port 53 (UDP) is blocked, DNS requests cannot be sent, resulting in resolution failure.

3. The default DNS server is unreachable or invalidly configured. Some Hong Kong servers default to using the DNS provided by your carrier, but these DNS servers are blocked or limited in certain routing segments, resulting in timeouts and unresponsiveness.

4. resolv.conf is automatically rewritten (Linux systems). In Linux, /etc/resolv.conf is the core file for DNS resolution configuration. However, if NetworkManager is enabled on an upgraded system, this file may be automatically overwritten with incorrect or empty values during system startup or network restart.

5. Network adapter driver issues. Some upgraded driver versions are not fully compatible with network cards, resulting in a semi-failed network. Although a physical connection exists, DNS packets may not be sent or responses may not be received.

6. The DHCP service is abnormal or not running. If the server uses DHCP to obtain configuration, the upgrade may cause the DHCP client to be inactive, preventing automatic DNS server information acquisition.

2. Specific solutions and practical operations

For the common reasons mentioned above, we provide solutions one by one, taking Linux as an example.

Method 1: Edit the resolv.conf file

sudo vi /etc/resolv.conf

Add or modify the following:

nameserver 223.5.5.5
nameserver 114.114.114.114

Save and the settings take effect immediately.

Note: If NetworkManager or systemd-resolved is installed on your system, this file may be overwritten after restarting the network.

Method 2: Configure a permanent DNS (for Debian/Ubuntu)

sudo vi /etc/netplan/01-netcfg.yaml

Add the following DNS configuration:

nameservers:
  addresses:
    - 223.5.5.5
    - 114.114.114.114

Then apply the configuration:

sudo netplan apply

Method 3: Check firewall settings

Check whether UDP port 53 is blocked:

sudo iptables -L -n | grep 53

If a DROP rule is found, execute:

sudo iptables -I INPUT -p udp --dport 53 -j ACCEPT
sudo iptables -I OUTPUT -p udp --sport 53 -j ACCEPT

III. Precautions Before Upgrading Your System

Back up your current DNS configuration: Before upgrading your system, back up /etc/resolv.conf or the Windows DNS configuration for quick recovery.

Record your current network card configuration: Take screenshots of your static IP, gateway, and DNS information in advance.

Test your backup DNS: Use tools like ping and dig to pre-test the speed and availability of multiple DNS servers.

Create a rollback snapshot: If your server supports snapshots, take a snapshot before upgrading to facilitate quick recovery.

After a Hong Kong server upgrade, DNS resolution issues may appear to be network issues, but they are often caused by misconfiguration, firewall blocking, or service anomalies. To prevent such issues from recurring, we recommend performing configuration backups, network monitoring, and troubleshooting drills during daily operations to enhance the server's fault tolerance and resilience. While Hong Kong servers offer cross-border advantages, they also require meticulous local configuration and a stable DNS strategy to achieve true high availability.